Methods for manufacturing a general GaN white light emitting device are generally classified into two methods, one is a method using a single chip, in which a white light emitting device is obtained by further forming a phosphor layer on a blue light emitting device or a UV light emitting device, and the other is a method using a multi-chip, in which two or more light emitting devices are combined to obtain a white light emitting device. The white light emitting device is exemplified by a white light emitting diode (LED), but is not limited thereto.
A representative method to realize a white light emitting device in the form of a multi-chip is manufactured by combining three R, G and B light emitting devices. However, the multi-chip type light emitting device has a drawback in that respective chips have non-uniform operation voltages and outputs of the respective chips are varied depending on a surrounding temperature, so that a color coordinate is changed. Due to the above drawback, the multi-chip type light emitting device is suitable for a particular lighting needing to display various colors because it controls the respective color light emitting devices rather than realizes the white light emitting device, but is not suitable for the white light emitting device.
Owing to the above reason, a binary system having an easy fabrication and superior efficiency is representatively used for realizing the white light emitting device. The binary system allows white light to be emitted by combining a blue light emitting device with a yellow phosphor layer, which is excited by the blue light emitting device to emit yellow light.
In detail, the binary system is a light emitting device, which uses the blue LED as an excitation light source and excits YAG (Yttrium Aluminum Garnet) phosphor using rare-earth 3-valent ion of Ce3+ as an activator, i.e., YAG:Ce phosphor using an excitation light emitted from the blue LED.
Also, the white light emitting device employs various packages according to its applications, and representatively includes a surface mounting device (SMD) type ultra-miniaturized light emitting device used in a backlight of a handheld terminal, and a vertical lamp type light emitting device used for an electronic board, a solid display device or an image display.
Index for analyzing the characteristics of white light includes a correlated color temperature (CCT) and a color rendering index (CRI).
In detail, the CCT indicates a temperature of an article when the article shines with emitting visible rays, it seems that a color of the article is identical to a color that a black body radiates at a temperature and it is assumed that the temperature of the black body is equal to that of the article. As CCT increases, the light dazzles a human being and becomes a bluish white. Therefore, in spite of identical white lights, when the CCT is low, people feel more warm, whereas when the CCT is high, people feel cold. Accordingly, by adjusitng the CCT, it is possible to meet even the specification of a particular lighting requiring various color feelings.
In a conventional white light emitting device using YAG:Ce phosphor, since the CCT is fixed only to 6000-8000 K, it is impossible to display various color feelings by adjusting the CCT.
The CRI indicates a degree that the color of an article is changed when sun light or artificial light is irradiated onto the article. When the color of the article is identical to that under sun light, the CRI is defined 100. In other words, the CRI is an index to show how the color of the article under the artificial lighting is close to that under sun light, and has a value of 0 to 100. Accordingly, as the CRI approaches 100, i.e., white light, people can feel that the color of the article under the artificial lighting has no difference than that under sun light.
At present, an incandescent lamp has a CRI of more than 80 and a fluorescent lamp has a CRI of more than 75, while a white LED using YAG:Ce phosphor has a CRI of approximately 70-75, which is low relative to those of the incandescent lamp and the fluorescent lamp.
Accordingly, it is problematic that the white LED using the conventional YAG:Ce phosphor is narrow in CCT and relatively low in CRI.